European patent EP 0664683 B1 discloses an electro encephalic neurofeedback apparatus. The apparatus comprises detecting means for detecting a bioelectric EEG signal including a set of bandwidths each having a range of one Hz around a selected frequency. The apparatus furthermore comprises sound means including respective music scores for each of said bandwidths. The sound means are configured for increasing said respective music scores in response to an increase in amplitude of said bandwidths as to increase the loudness of the music played to the patient.
Since the sound means raise the music's loudness in response to an increase of amplitude for some bandwidth, the patient is made well aware of the procedure to induce relaxation. Such awareness is not beneficial; in fact it is counterproductive as it may bring about additional stress. Therefore a problem of the apparatus is in its limited capability to effectively reduce a patient's stress level.